Air swept ball mill



April 23, 194.6. w. L MAxsoN ETAL AIR SWEPT BALL MILL Filed sept. e, 1943 a frusto-conical screen Patented Apr. 23, 1946v Walter L. Maxson, Milwaukee, Wis.,

nant, Salt Lake City, Baxa,

John A. Fag- Utah, and Edward Il.

Milwaukee, Wis., Chalmers Manufacturing kee, Wis., a corporation of assisnors to Allis- Company, Milwau- Deiaware l Application September 6, 1943, Serial No. 501,372

7 claims. (c1. 241-4'1) This invention relates to air swept ball mills.

Air swept ball mills for grinding dry materials are well knovm in the art. It is also well known to provide a discharging chamber in such mills distinct from the grinding space proper.

An object of this invention is to provide a simple, improved discharge and classifying chamber for air swept ball mills, having a diaphragm and frusto-conical screen for separating and discharging fine and coarse materials.

It is a further object to improve the action of in an air swept mill by fluting its external surface.

It is a still further object of this invention to divide an air swept ball mill discharge chamber into a material cascading part and an oversize discharging part and to provide means for deilecting cascaded oversize material into saidoversize discharging section by use of a' truste-conical screen surrounding the main air current.

Other and still further objects may appear from the following description with reference to the accompanying drawing, in which:

Fig. 1 is a vertical longitudinal sectional view of a mill according to the invention; and- Fg. 2 is Fig. 1. t

Like reference characters refer to similar parts throughout theseveral views ofthe drawing.

An air swept ball mill I has a feed end wall 2 with hollow trunnion 3 and a discharge end wall a cross-section on the line lI-I of 4 with a hollow discharge trunnion 5. The mill is rotatably supported by trunnons 3 and 5 in bearings 6. A stationary air inlet hood 'I is rotatably sealed in known manner with trunnion 3. A

raw material feeder 8 of known type extends through hood 'l and trunnion 2. A gaseous fluid and material conduit or offtake 9 makes a rotatable seal connection of suitable known type with discharge trunnion E. Any known means of suitable type, not shown, are provided to draw a current of air or other elastic fluid through the mill I and conduit or oiitake 9.

The interior of mill I is divided by a forarninous grid or screen II having a large central aperture I2 into a grinding space I3 and a discharge and classifying chamber I 4.

Within the chamber I4 and connected with the grid II adjacent its' central aperture I2 is a funnel-like, frusta-conical perforated screen I6 coaxial with the mill I with its smaller end forming a tubular liner Il extending through the discharge trunnion 5. The portion of the liner extension Il within the trunnion is imperforate, and is coaxially spaced from the inner surface of the trunnion by a helical iin I8. An annular diaphragm I9 is peripherally secured within the space I4, in spaced relation to grid I I, end Wall 4, and screen I6. Circumferentially spaced between the grid II and diaphragm I9 are a plurality of lifter members 2l. The part of the constant discharge chamber I4 lying axially between grid II and diaphragm I9 may be referred to as a ground material receiving or cascading part, and the part lying axially between diaphragm I9 and end wall 4, which need not be provided with lifters, may be referred to as a coarse material or oversize discharging part. Outlets 22 provided in the mill shell between diaphragm I9 and end wall 4 provide an escape for material in the oversize discharging part, and may be connected by helical conduits 23 and openings 24 with the grinding `face longitudinally fluted or provided with ridges 26 along elements of the surface. The perforations 2l of the screen may be arranged in rows between the ridges, and are preferably of increasing area toward the smaller end of the screen I6 for a reason to appear hereinafter.

In-operation the mill is rotated clockwise as viewed in Fig. 2. A current of air is induced through the hood I and grinding space I3. A strong current of air will sweep through the central aperture I2 of grid II and will ow with increasing velocity through the funnel shaped screen I6 and the liner extension IT. Flow through the outer openings of the grid II and chamber I4 outside due to the greater obstruction to flow through limited orices.

Material to be comminuted, such as rocks, ores. coal, etc., is fed through feeder 8 into the grinding space I 3 which may contain a charge of steel balls or other known grinding media. The raw material being tumbled in the mill is comminuted by the well known action of ball mills. Some of the finest powdered material will be swept along with the current of air through grinding space I3.-

ceiving part of chamber I4 and will be lifted and The fin I8 is pitched to screen I6 will be negligible nected with said discharge screen,

cascaded by lifters 2i over the outer surface of funnel-like screen It. The nner portion of this cascading material will pass in fine streams through the perforations 21 of screen l5 and be brought into intimate contact with the stream of air passing with increasing velocity through screen i5 and liner extension il. The strong current oi air will carry away substantially all ground material of suilcient neness, coarser materials passing all the way through. Of the materials cascaded over screen IB a portion, largely oversize, will be deflected through the central opening of diaphragm I9 by the incline of the screen surface. This coarser the annular oversize discharging part of chamber l.

Any material too coarse to be carried all the way out of oitake 9 settling and tending to accumulate in the oitake at its seal with trunnion 5, will be conveyed into the oversize discharging portion of chamber Id. All material in the material discharging part of chamber will be dis- `charged into conduits 23 and returned to the grinding space i3. s

The fluting of screen i6 tends to increase its capacity and aid classiiication by impeding travel of material circumferentially of the screen and promoting a longitudinal sliding of the material over the screen surface so that more of the material may be dispersed through the perforations 21 which are of greater area as the air velocity Vwithin the screen increases and the stream is able to carry more material. Coarse material tends to be more 'effectively deected through the opening of diaphragm I9 due to this longitudinal sliding promotedby iluting the surface of screen I6.

While a particular embodiment of the invention has been shown and described in detail, it will be understood that the invention is intended to include such modifications and equivalents as may readily occur to a person skilled in the art, and is limited only by the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:A

l. A rotating air swept ball mill having a discharge end, a transverse discharge screen spaced from the discharge end thereof, separating said mill into a grindingV chamber and a discharge chamber. a hollow discharge trunnion, a liner extending through saidtrunnion in coaxially spaced relation, and having a frusta-conical, perforated extension within said discharge chamber con an annular diaphragm peripherally dividing said discharge chamber and having its inner border coaxial with and spaced from the surface of said frusto-conical extension, material lifting means in said discharge chamber between said annular diaphragm and said discharge screen. and means communieating with said discharge chamber adjacent the discharge end of said mill to return oversize from said discharge chamber to the grinding chamber of said mill.

2. In an air swept the discharge end of said mill, a central aperture in said grid, a discharge trunnion, a coaxial liner radially spaced within said discharge trlmnion, a helical spacer on said liner extending between said liner and the inner surface of said trunnion. a. perforated frusta-conical extension on said liner having its base secured to said grid adjacent the central 'opening thereof, an annular wall peripherally dividing the spacenbetweensaid grid and material will enter :recaen the discharge end of said mill and having its cen-V trai aperture spaced from said liner and said frusto-conical extension, material lifting means in the space between said grid and said annular wall, means for circulating elastic fluid through said trunnion liner and mill for carrying away nely ground product, and oversize discharge means connected with the space bounded by said liner, said annular wall and the discharge end and trunnlon of said mill.

3. An air swept ball mill having a grinding space and a discharge trunnion, comprising a grid spaced from having a Acentral aperture, an annular wall peripherally dividing the space between said grid and said discharge trunnion, material lifting means between said. grid and said wall,- coaxial screening means connected with said grid and said trunnion, so constructed and arranged as to separate material showered externally `thereon' into ilnes passing within said coaxial screen and oversize deected by said screen through the inner aperture of said annular wall, means between said wall and trunnion for returning said oversize' to said grinding space, and means for circulating elastic uid through the interior of said mill, screen, and trunnion for removing said fines.

4. In an air swept ball mill having a grinding space and a hollow discharge trunnion, a constent discharge chamber comprising an annular foramious grid spaced from the discharge end of said mill, a frusto-conical screen in said chamber having its base secured in the central opening of said annular grid and its smaller end connected to a cylindrical liner in said hollow discharge trunnion, an annular diaphragm in said chamber secured to said mill periphery and having its inner opening spaced from the surface of means operatively connected said screen, and lifting means in said chamber for showering material passing said foraminous grid upon the external surface of said screen.

5. An air swept ball mill comprising a substantially horizontal drum rotating on its axis, a transverse discharge chamber in said drum, a frusta-conical screen within said chamberand coaxial with said drum, an annular transverse diaphragm riphery radially spaced from and surrounding said screen adjacent the smaller end thereof, means in said chamber for showering ground material over theexternal surface of said screen,

said screen deecting oversize particles axiallythrough the opening of said diaphragm.

6. A horizontal dry classifier for ball mills comprising a frusta-conical screen having a coaxial Itubular extension at the smaller end thereof, with said screen for rotating said screen on its axis, means operatively connected with said tubular extension for causing a current of air axially through the interior of said screen and said tubular extension, an annular transverse wall having an opening sur- 65 ball mill, a grid spaced from l rounding said screen in radially spaced relation adjacent the smaller end thereof, and means for showeringground material transversely over the external frusto-conical screen surface of said screen, the frusta-conical surface of said screen being effective to deflect oversize ground material particles axially through said opening.

7., A horizontal drysclassifier for granular materials comprising' a perforated frusta-conical screen element mounted for rotation with itsaxis horizontal, the`perforations in said scr'een increasing in total' area of openings in successive said discharge trunnion and.

in said chamber having its inner pe-l aasaoex y 3. axial bands from the larger end to the smaller end means for showering granular materials to be separated transversely over the external surface of said screen element, and means operatively connected with said screen for causing a generally axial stream of gaseous iiuid to ilow through the interior of said screen element tolward and outwardly through the smaller end,

said screen dispersing ever larger quantities of fine granular materials passing through its perw forations in the path of the stream of gaseous iiuid through said screen as the velocity and hence the capacity of said stream of gaseous fluid to carry off such materials is increased due to the gradual constriction of the passage through `the interior of said screen. l

WAL'I'ER. L. MAxsoN. JOHN A. FAGNANT. EDWARD H. BAXA. 

